1. Field of the Invention
The present invention relates to a method for forming a ceramic film on the surface of a base material.
2. Description of the Related Arts
There is a method for forming a compact by casting molten metal or slurry into a ceramic mold. It is known in this method that, after the surface of a pattern made from wax, polystyrene or urea has been coated with ceramic powder, said pattern is removed. Said pattern is dissipated by being heated and melted or by solvent extraction. A ceramic film, with which the pattern has been coated, is used for a mold. As the ceramic powder, mullite, zircon, fused silica or alumina is pointed out.
A method wherein a porous compact is converted to a dense sintered body is disclosed in a Japanese Patent Publication Laid Open No. 146205/79. The surface of a compact made from metallic powder or ceramic powder is coated with ceramic powder. The compact having a film of ceramic powder on the surface thereof is heated and sintered. The film of ceramic powder is converted to a gas-impermeable dense film. The compact with said film is subjected to a hot isostatic hydraulic pressing process. As the ceramic powder, borosilicate glass, high-silicate glass, silica glass, silicon nitride, alumina or boron nitride is pointed out.
It is also known that a film of ceramics or ceramic precursor is formed on the surface of a base material to change the surface properties of the base material.
In those methods, ceramic powder is dispersed in water or organic solvent. A slurry thus prepared is regulated by adding inorganic or organic binder to the slurry if necessary. A pattern or a compact, which is dissipated by being heated and melted or by solvent extraction, is dipped into the slurry, and the surface of the pattern or the compact is coated with the slurry. After the pattern or the compact has been coated with the slurry, the pattern or the compact coated with the slurry is dried. It is also known that the coating of the pattern or the compact with the slurry and the dip of the pattern or the compact into the slurry are alternately carried out.
In the method wherein the base material is dipped into the slurry, the ceramic powder insufficiently adheres to the base material, and the density of the film becomes unequal. On the other hand, in the case of dipping the base material into liquid of the precursor of ceramics, the precursor of ceramics insufficiently adheres to the base material, and air having adsorbed to the surface of the base material is taken in the film. In the case of dipping the base material whose surface is coated with slurry into a fluidized layer of ceramic powder, the density of the film becomes unequal because ceramic powder is liable to aggregate in the fluidized layer of ceramic powder. For the aforementioned reasons, cracks are often generated on the surface of the base material at the successive step of heat treatment.